Brace for a topsy-turvy summer

Megan Levy

It's a summer weather forecast to make holidaymakers break into a cold sweat.

Parts of Australia's east coast, particularly New South Wales and Queensland, are expected to be drenched in higher-than-average rainfall this summer due to the strengthening of La Nina weather conditions.

Cooler temperatures are expected to accompany the downpours, with the mercury forecast to dip slightly below the summer average in those states. Northern Australia is also at increased risk of tropical cyclones during La Nina.

Weatherzone senior meteorologist Josh Fisher said this year's La Nina event was not predicted to be as severe as the conditions in the 2010-11 summer, which saw Queensland and Victoria lashed by devastating rain and floods, but it would still have a noticeable impact through the next three months.

In Victoria, long-term-average temperatures and rainfall are expected to remain about average across the three months of summer.

But Victoria's topsy-turvy start to the season - with hot days interspersed with thunderstorms and chillier conditions - is set to continue throughout the next three months. It will be accompanied by higher humidity levels, Mr Fisher said.

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"Rainfalls are expected to stay close to average or slightly above in Victoria, which will also keep temperatures close to average, but this doesn't rule out those extreme heat events where we see those temperatures climb up to around 40 degrees. Like we've already seen during these first two weeks of summer, there's going to be a large variation," Mr Fisher said.

Mr Fisher said increased thunderstorm activity was also expected to strike right across Australia's east coast due to La Nina.

"This is a feature that we're likely to see right throughout the summer, including anywhere from South Australia to Victoria, News South Wales and Queensland," Mr Fisher said.

During the first four days of December daytime temperatures in Melbourne remained below 21 degrees, the coolest start to summer in 24 years.

The temperature rebounded during the second week of summer, with the mercury climbing as high as 32 degrees on December 8, followed by rain and thunderstorms. The city received 34 millimetres of rain in just 12 hours from 5pm on December 10, more than half of the December rainfall average.

Mr Fisher said during a La Nina event, sea surface temperatures near South America cooled below average while sea surface temperatures close to Australia warmed up.

That affected atmospheric circulation, leading to a strengthening of easterly trade winds over the tropical Pacific.

The winds gained large amounts of water over the Coral Sea, which were then pushed over eastern parts of Australia.

Meantime after a wet start to December, Western Australia is forecast to receive average rainfall and temperatures for the remainder of the summer.

Severe storms have dumped 76 millimetres on Perth already this month, well above the long-term average of just six millimetres - making it the wettest December in 50 years, Mr Fisher said.

He said that above average-rainfall was not linked to La Nina, but a warming of sea temperatures off Western Australia. Rainfall was expected to be around the long-term average of 10 millimetres a month or slightly more for the rest of summer, Mr Fisher said.

"Western Australia has seen a lot of rain so far this month but that's really been heavy rain over a day or two. Summer is usually their drier months," he said.